Process of oxidizing carbon monoxide and hydrocarbons in exhaust gases



PROCESS OF OXIDIZING CARBON MONOXIDE AND HYDROCARBONS IN EXHAUST GASESAugustus H. Batchelder and Robert P. Sieg, Berkeley, Calif., assignorsto California Research Corporation, San Francisco, Calif., a corporationof Delaware No Drawing. Filed June 10, 1957, Ser. No. 664,473

1 Claim. (Cl. 232) This invention relates to an improved exhaustoxidation system for internal combustion engines whereby the dischargeof carbon monoxide and of unburned and partially oxidized products tothe atmosphere is prevented or minimized. V r 1 When fuels burn in anautomotive engine, combustion is relatively incomplete and aconsiderable portion .of the fuel is discharged in the unburned orpartially burned condition. Various means have been proposed heretoforefor ameliorating this situation, such means generally involving passingthe exhaust gases through a bed of oxidation catalyst in the exhaustline. For example, US. Patent No. 1,400,959 proposes the use ofgranulated coke, powdered charcoal and alumina in finely divided form,the catalyst mass being used as a resistance heater by passing electriccurrent from an Accordingly, if full combustion of the exhaust productsis to be eficcted, oxygen must be supplied to'the exhaust either by anair blower (as in US. 1,903,803) or by a Ventun' eductor as in thepresent application.

This oxygen or air deficiency varies greatly with-different operatingconditions; Thus, during normal or cruising conditions, approximately 1to'4% of thefuel supplied to the engine is discharged unburned. Thisamount increases to from about 2 to 8% while the engine is idling, ormore in traflic (intermittent start and stop conditions) and may be aslarge as 50% during start-up or under rapid decelerating conditions.While it would be possible to continuously supply the exhaust line withan excess of air large enough to efiect complete combustion duringperiods of normal as Well as abnormal (traflic and rapid deceleration)operation, such operation would be inefiicient'and uneconomical inasmuchas the periods of high air demand are relatively brief and usually occurduring only a small proportion of the total operating cycle. Further,the use of unnecessarily large amounts of extraneous air during periodsof normal operation would so lower the temperature of any oxidizingcatalyst employed as to tend to make the catalyst inefiective.

Accordingly, it is an object of the present invention to provide a meansof oxidizing undesired components of exhaust gases which minimizes theuse of extraneous air.

It has now been found the desired effect can be secured by employing asoxidation catalyst a polyvalent acid oxide of a metal in groups Vthrough VIII of the periodic table, together with the salts of saidoxides, preferably alkali and alkaline earth metal salts thereof.

ice

Patented June 28, 1 960 The metal in such compounds varies in valencestate and the compound thus in oxygen content, depending on whether thecompound is subjected tooxidizing or reducing conditions. When subjectedto contact with air at elevated temperatures, they take on an elevatedvalence and (in effect) store oxygen. When subjected to contactwith-reducing gases such as carbon monoxide or hydrocarbons (in theabsence of suflicient extraneous air) such compounds become reduced tolower valence states and thus liberate oxygen which serves to oxidizethe reducing gases. The catalyst thus serves as an oxygen reservoir tobe drawn on when the-supply of added air is insufiicient to maintain thecatalyst in the more fully oxidized state. This makes it possible to sooperate as to supply only an average amount of extraneous air to aninternal combustion engine exhaust line, and to depend on the catalystitself for oxygen during periods of abrupt and brief, peak oxygendemand, the said average extraneous air supply being maintained at suchalevel that the catalyst becomes reoxidized during normal and steadyoperation of the engine.

The polyvalent metal-containing catalyst employed in the oxidationchamber should be one of high surface area and which is characterized bygood activity evenafter repeated exposure, over relatively longoperating periods, to the high temperatures of exhaust gases.Representative catalysts, which are generally supported on a basematerial of high surface area such as'activated alumina, silica, or thelike, are M00 W ,;VO V205, T3205, Nah [1104, Na M'nO K2MI104,Ca(MI10'4)2, Ba(Mn0 N212MOO4, K MoO Na WO K WO N-a VO Na20l'O4,Nagcl'zoq, CaCrO; and KzCI'zOq. In general, the oxidation catalyst ispresent in an amount of from about 2 to 30%, based on the overall weightof the catalyst and its support.

A particularly useful oxidation catalyst was obtained by impregnating anactivated alumina of high surface area with potassium dichromate. Thepore volume of the support was determined by Water displacement. A watersolution of potassium dichromate was then prepared of such concentrationthat the quantityof absorbed solution would result in a finishedcatalyst containing 14 wt. percent of potassium dichromate. The catalystwas then dried at 250 F. and calcined for four hours at 1,200 F.

This catalyst was somewhat active at about 700 F and is fully active atabout 900f F., as heatedby theexhaust gases. In the absence of an excessof air over that stoichiometrically required to completely burn theexhaust gas components, the chromium catalyst becomes reduced to thetrivalent form and thus releases a substantial amount of oxygen to thecombustion process. By thus maintaining a reservoir of oxygen for useduring periods of abnormal engine operation, as under deceleratingconditions, it is possible to effect full combustion of exhaustcomponents while supplying less. excess air to the exhaust stream thanwould otherwise be required. This feature also characterizes thecatalysts enumerated above. Thereafter, as normal engine operation isresumed, with an exhaust stream relatively rich in free oxygen, thecatalyst reverts to the K Gr O state, thereby resuming the dual functionof an efiective oxidation catalyst and an oxygen reservoir. Thiscatalyst was in the form of W -inch pellets. However, larger or smallercatalyst particles can be employed, as desired, provided flow of gasesthrough the catalyst body is not unduly restricted.

In carrying out the present invention, extraneous air is, as notedabove, introduced into the exhaust manifold by the use of a venturi orother air supplying device at a point upstream from the catalyst chamberwhich may replace the conventional mufiler or supplement the action ofthe same. The quantity of air so introduced is proportioned to providethe exhaust line with an amount of free oxygen which is at least thatstoichiometncally required to burn completely the carbon monoxide andhydrocarbons contained in the exhaust gas under normal (i.'e., cruising)conditions of operation. More specifically, good results are obtainedwhen the quantity of air is sufiicient to provide an amount of oxygenequal to about 100 to 200% (preferably 125 to 150%) ofsaidstoichiometrical requirement.-

The amount of catalyst to be employed for a given engine will varydepending on the size of the engine and upon the length of the desiredservice period. However, with a 14% K Cr O alumina catalyst of the typedescribed above, for example, it is considered that an I amount of fromabout 10 to 25 pounds thereof will be adequate for the averageautomobile engine. It is contemplated that the operator will be able todetermine whether or not the catalyst is functioning efficiently (as bythe use of a temperature indicating device responsive to the exothermicheat generated by an efficient catalyst, for example), and that when thecatalyst gives evidence of wearing out, replacement thereof may be made.This may conveniently be accomplished by providing the catalyst in theform of a cartridge which can be removed for inspection and/orreplacement at will.

The present invention is illustrated by the following examples.

Example I V was oxidized, but at a rate too slow to be practical. A muchmore promising rate was achieved at 700 F.

(3) One mol of dichromate makes available 1.5 mols of oxygen as itisreduced to the Cr O 'state. When'a stream of butane in nitrogen wasoxidized in the absence of extraneous air, more than 90% of the paraflinwas converted to CO until about half of the dichromate was reduced. Therateof conversion then fell off rapidly,

though the butane continued to be oxidized until the 'dichromate wassubstantially all reduced.

(4) Addition of 1.5 times the stoichiometric amount of oxygen caused thebutane to be completely oxidized during a period in which sufiicientbutane was passed over Example 11 A chamber containing about 12 poundsof the same potassium dichromate catalyst as employed in Example I, wasdesigned to replace the muffier of a 1954 Chevrolet 6, was introducedinto the exhaust stream by means of a venturi upstream from the mufller,the :air so introduced being about 15% by volume of the resultingexhaust stream. It was found that the catalyst was highly eifective inoxidizing the hydrocarbons present in the exhaust under varyingoperating conditions. Thus, at a catalyst temperature of about 1,000 Fnearly quantitative combustion of the hydrocarbons was achieved underidling conditions. About two-thirds of the hydrocarbons were burned at30 miles per hour, and a further reduction in noncombusted gases couldhave been obtained by increasing the volume of catalyst. The catalystchamber did not produce excessive pressure drop in the exhaust systemeven under full-throttle conditions. The products of the combustion oftetraethyl lead in the gasoline did not poison the catalyst. The chamberfunctioned without being supplied with external heat. However, in normalcity driving at low speed it might be desirable to initiate the reactionwith external heat, which could be cut oif after the chamber began tofunction. Since the reaction is exothermic, the catalytic combustionwould :then be selfsustaining.

chromate disposed on an activated alumina support.

References Cited in the file of this patent FOREIGN PATENTS 270,541Great Britain May 12, 1927 312,200 Great Britain May 21, 1929 413,744Great Britain July 26, 1934

